17 December 2009

ALIEN 'WATER WORLD' FOUND

Astronomers say they have detected a planet just six and a half times as massive as Earth - at a distance so close its atmosphere could be studied, and with a density so low it's almost certain to have abundant water.

The alien world known as GJ 1214b orbits a red dwarf star one-fifth the size of our own sun, 40 light-years away in the constellation Ophiuchus, the astronomers reported in Thursday's issue of the journal Nature.

"Astronomically speaking, this is on our block," David Charbonneau of the Harvard-Smithsonian Center for Astrophysics, lead author of the study, told reporters this week. "This is a next-door neighbor. For perspective, our own TV signals have already passed beyond the distance of this star."

He said the planet was detected using an array of eight off-the-shelf, 16-inch telescopes equipped with commercially available cameras.

"Since we found the super-Earth using a small ground-based telescope, this means that anyone else with a similar telescope and a good CCD camera can detect it too," Charbonneau said in a news release. "Students around the world can now study this super-Earth."

Super-Earths - planets that are roughly two to 10 times Earth's mass - represent the hottest frontier in the years-long search for worlds beyond our solar system. Planet-hunters reported finding their first transiting super-Earth in February, and earlier this week, other researchers addedtwo more super-Earths to the list.

Those planets orbit stars like our own sun, but the brightness of GJ 1214b's parent star is hundreds of times dimmer. The planet is also much closer to the star than any of our own solar system's planets, orbiting at a distance of only 1.3 million miles (2 million kilometers). That combination suggests that the planet's surface temperature would be about 400 degrees Fahrenheit (200 degrees Celsius), Charbonneau's research team reported.

Charbonneau speculated that GJ 1214b was a little too hot for life as we know it, "but it didn't miss it by very much."

The planet's discovery was hailed as a potential breakthrough by Geoffrey Marcy, an astronomer at the University of California at Berkeley who is a pioneer in the planet quest. In a commentary written for Nature, Marcy said Charbonneau and his colleagues "provide the most watertight evidence so far for a planet that is something like our own Earth, outside our solar system."

How it was found
GJ 1214b was detected thanks to an innovative telescope system, a cleverly focused observation campaign - and perhaps a little bit of luck. The eight-telescope array, dubbed the MEarth Project, was set up at the Whipple Observatory on Mount Hopkins in Arizona. The telescopes were programmed to gaze at 2,000 low-mass stars and check for slight, regular dips in light that could be caused by a dark planet's transit across the star's disk.

Relatively dim, relatively close stars were favored because the planet's dimming effect would be more noticeable than it would be with brighter, bigger, farther-out stars.

Just a few months after the MEarth Project began, graduate student Zachory Berta spotted the signature of GJ 1214b's 38-hour orbit. Based on the pattern of the dimming, the team figured out that the planet was 2.7 times as wide as Earth.

The astronomers then turned to another instrument, the HARPS spectrometer on the European Southern Observatory's La Silla telescope in Chile, to figure out the planet's mass. Such mass calculations depend on another technique that checks for the slight wobble in a star's motion caused by a planet's gravitational pull. The HARPS observations indicated that the planet was 6.55 times as massive as Earth.

Putting those measurements together, the team was able to model the planet's density and composition. The best fit for the data was a mixture consisting of about three-quarters water and other ices, one-quarter rock and a gaseous atmosphere.

Implications of a water worldAlthough the surface temperature on GJ 1214b would be well above water's boiling point on Earth's surface, Charbonneau said the planet could nonetheless possess an exotic form of liquid water due to extreme atmospheric pressure at the surface. In today's news release, Berta said the pressure may turn at least some of the water into a rare crystalline form known as ice-seven.

"Despite its hot temperature, this appears to be a water world," Berta said.

On Earth, organisms have been found living near deep-sea hydrothermal vents, where superheated water is held under high pressure. But Charbonneau said he wouldn't want to bet that life could endure under GJ 1214b's crushing conditions.

In fact, it's too early to bet heavily on any detailed description of GJ 1214b. Fortunately, Charbonneau said, the star is close enough that the Hubble Space Telescope could someday analyze the composition of the planet's atmosphere. "That will make it the first super-Earth with a confirmed atmosphere - even though that atmosphere probably won't be hospitable to life as we know it," he said.

Knowing what the atmosphere is made of, and how thick it is, could help astronomers determine whether their characterization of GJ 1214b as a water world is correct. "It's possible that what you have is a ball of rock with a much bigger envelope of light gas," Charbonneau said.

The larger implication of the Nature study is that other super-Earths may be waiting out there with just the right conditions for life. "We found this planet in the first six months," Charbonneau noted. "We had only looked at a small fraction of the stars that we planned to look at through the entire project. That means that either we got really lucky - which is possible - or these planets are common."

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